Numerous means have been sought to improve the fuel-efficiency of moving bodies, and especially moving bluff bodies, by reducing their aerodynamic drag. In the field of surface transportation, and particularly in the long-haul freight industry, even small improvements in fuel efficiency can reduce annual operating costs significantly. It is therefore advantageous in the design of a vehicle to reduce drag forces, thereby increasing the aerodynamic properties and efficiency of the vehicle.
The over-the-highway cargo hauling tractor-trailer combination is one vehicle that experiences excessive aerodynamic drag. Generally described, tractor-trailer combinations typically include a tractor having a so-called fifth wheel by which a box-like semi-trailer may be attached to the tractor by an articulated connection for transportation of the semi-trailer. The front profile of a conventional tractor is typically a bluff body, producing significant pressure drag at typical highway speeds. One reason for the large pressure drag is the presence of a sharp angle located at a leading edge of the tractor hood. More specifically, typical tractor front sections include a substantially vertical front surface or grill that meets, along an upper edge, a substantially horizontal top surface. The air flow passing over the front section, therefore, must negotiate an abrupt change in direction as the edge where the hood structure transitions from a substantially vertical orientation to a substantially horizontal orientation. This abrupt turn causes the flow to ‘separate’ from the top surface of the hood, forming a highly turbulent region of air located directly above the top surface of the hood, between the leading edge and the windshield.
Referring to FIG. 1, a perspective view of a prior art Class 8 tractor 10 showing an air stream 12 flowing over a hood 16 is depicted. The depicted air stream 12 encounters the conventionally shaped Class 8 tractor 10 at the substantially vertical surface of the front surface or grill 14 of the hood 16. (It will be appreciated that for purposes of the present aerodynamic discussion, the tractors 10 forward motion at highway speeds is equivalent to an air stream 12 having a similar but opposite velocity flowing over a stationary tractor.) The air stream 12 turns upwardly as it negotiates the grill 14, and separates at a leading edge 15 of the hood 16, thereby forming a vortex or wake region 22 located aft of the leading edge 15. The airflow separation at the leading edge 15 causes the formation of a large wake region 22 and pressure losses due to eddy formation in the wake region, thereby increasing drag on the vehicle.
Previous investigations for reducing aerodynamic drag of tractor-trailer combinations, including reducing the bluff body characteristics of the conventional Class 8 tractor 10, resulted in streamlining the outer contours of the front section of the tractor 10, widespread adoption of fixed air deflectors mounted on the roofs of tractor cabs, and wholly redesigned tractors that utilize aerodynamic fairings to gradually increase the relatively small frontal area of the tractors to match, and to blend smoothly with, the larger cross-section of typical trailers. For example, in order to reduce abrupt changes in air flow over the hood, some modem tractor hoods have been made to slope downwardly from the windshield toward the front of the tractor, creating a less abrupt transition between the front grill 14 of the front section and the top surface of the hood 16. This more aerodynamic shape reduces the amount of flow separation, and consequently reduces the pressure drag exhibited upon the vehicle. However, such a design may still incorporate discontinuous regions, due to packaging for under hood components such as radiators, air ducting, or coolant tanks, that produce abrupt changes in air flow resulting in the creation of a wake region 22, again an increase in drag.